- Diacyl Disulfide: A Reagent for Chemoselective Acylation of Phenols Enabled by 4-(N,N-Dimethylamino)pyridine Catalysis
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A general and excellent acylation reagent, diacyl disulfide, was uncovered for efficient ester formation enabled by DMAP (4-(N,N-dimethylamino)pyridine) catalysis. This protocol offered a promising synthetic platform on site-selective acylation of phenolic and primary aliphatic hydroxyl groups, which greatly expanded the realm of protecting group chemistry. The importance of the reagent was also reflected by its excellent moisture tolerance, high efficiency, and potential in synthetic chemistry and biologically meaningful natural product modification.
- Liu, Hong-Xin,Dang, Ya-Qian,Yuan, Yun-Fei,Xu, Zhi-Fang,Qiu, Sheng-Xiang,Tan, Hai-Bo
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p. 5584 - 5587
(2016/11/17)
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- Lipase catalyzed esterification of cresols
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Esters of m- and p-cresols with organic acids having carbon chain lengths C2-C18 have been prepared by using lipases from porcine pancreas and Rhizomucor miehei. Gram level conversions are carried out under non-solvent conditions in case of shake flask experiments and continuous removal of water at bench-scale levels. Addition of 0.1 mL of 0.1M phosphate buffer at pH 7.0 to the reaction mixture shows better conversions. Optimization studies have been carried out for p-cresyl laurate synthesis using Rhizomucor miehei lipase which show a maximum conversion of 74.4 %. Better conversions are obtained with larger amounts of enzyme. Porcine pancreas lipase catalyzed synthesis of m- and p-cresyl esters show that under identical reaction conditions acids with lower carbon chain lengths (C2-C4) give ester yields above 30%, while those with longer carbon chain lengths give ester yields 30%.
- Suresh Babu,Karanth,Divakar
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p. 1068 - 1071
(2007/10/03)
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- Effective Charge Development in Ester Hydrolysis Catalyzed by Cationic Micelles
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Reaction of hydroxide ion with substituted phenyl laurates is catalyzed by micelles containing CTAB (cetyltrimethylammonium bromide).The observed rate constants for ester consumption obey a Michaelis-Menten-like rate law, kobsd = (kOHKeq + kOHcat)/(Keq + ), where kOHcat is the second order rate constant for reaction of hydroxide ion with micelle-bound ester, Keq is a partition coefficient for dissociation of ester from the micelle-bound ester, and kOH is the second order rate constant for reaction of free ester with hydroxide ion.The value of the parameter Keq is independent of the nature of the substituent on the ester.The ratio kOHcat/Keq represents the free energy change from ester in aqueous solution to the transition state of the ester reaction in the micellar pseudophase.The dependence of log kOHcat/Keq on the pKa of the leaving phenol obeys a linear Bronsted equation with β1g = -0.51, and log kOH obeys a linear Bronsted equation with β1g = -0.56.Since both Bronsted parameters refer to catalyzed and uncatalyzed reactions starting from the same standard state (aqueous solution), the effective charge in the transtition state of both reactions is almost the same.The microscopic medium is thus similar for both cases, consistent with the COOAr moiety's residing in an aqueous-like region of the pseudophase in the catalytic process.
- Al-Awadi, Nouria,Williams, Andrew
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p. 2001 - 2004
(2007/10/02)
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- the Effect of Hydrophobic-Lipophilic Interactions on Chemical Reactivity. 1. New Evidence for Intermolecular Aggregation and Self-Coiling
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The hydrolytic behavior of 21 para-substituted phenyl esters of n-alkanoic acids (n-X) with various chain lengths in 60:40 (Φ=0.60), 50:50 (Φ=0.50), and 40:60 v/v (Φ=0.40) Me2SO-H2O mixtures was studied.Four new sets of experimental results which include departure from Hammett correlation for N-hexadecanoates (16-X), substantial differences in activation parameters, and effects of amylose on the kinetic parameters and on the activation parameters are presented along lines of reasoning mentioned by previous authors.These data establish that the aggregation and self-coiling of the n-alkanoate chain actually exist under experimental conditions.
- Jiang, Xi-Kui,Hui, Yong-Zheng,Fan, Wei-Qiang
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p. 3839 - 3843
(2007/10/02)
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